A) Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device which uses nitride semiconductor and has positive and negative electrodes disposed on the same side of a substrate.
B) Description of the Related Art
FIG. 15A is a cross sectional view of a semiconductor light emitting device disclosed in Japanese Patent Laid-open Publication No. 2003-224297. On a sapphire substrate 200, a buffer layer 201 is formed. On the buffer layer 201, an n-type contact layer 202 of n-type GaN, an n-type clad layer 203 of n-type AlGaN, a light emitting or luminous layer 204 of InGaN, a p-type clad layer 205 of p-type AlGaN and a p-type contact layer 206 of p-type GaN are stacked in this order. In a partial region, the layers from the p-type contact layer 206 to the surface layer of the n-type contact layer 202 are etched to expose a partial area of the n-type contact layer 202.
A p-side ohmic electrode 207 is formed on the p-type contact layer 206 and an n-side ohmic electrode 208 is formed on the exposed surface of the n-type contact layer 202. The lamination structure of these layers is covered with a translucent insulating film 210. Openings 210a and 210b are formed through the insulating film 210 to expose partial surfaces of the n-side ohmic electrode 208 and p-side ohmic electrodes 207, respectively.
A reflection film 211 is formed on the insulating film 210, covering the p-side ohmic electrode 207. A p-side pad 213 is formed on a partial surface of the reflective film 211, and an n-side pad 212 is formed on the n-side ohmic electrode 208.
The reflection film 211 is made of Al, Ag or Rh and reflects light radiated in the luminous layer 204 toward the substrate 200 side. Light passes through the substrate 200 to be output to an external. In order to improve a light output efficiency, it is desired to increase a reflectance of the reflection film 211. Ag has a very high reflectance in the range from ultraviolet light to visual light. Ag is, however, the metal likely to evolve electrochemical migration. According to “High Reliability Micro Soldering Techniques” by Tadashi TAKEMOTO and Ryouhei SATO (Kogyou Chousakai, Publishing, Co., Ltd), the Ag migration evolution mechanism is described as in the following.
As an electric field is applied under the existence of moisture, Ag is dissolved at the anode and hydrogen (H2) is generated at the cathode. Ag+ ions and OH− ions react near the anode and silver hydroxide AgOH is formed. Chemically unstable silver hydroxide AgOH is decomposed and colloidal silver oxide Ag2O is formed. Silver oxide further reacts to form Ag+ ions. These reactions are repeated so that Ag2O and Ag+ ions move to the cathode, Ag is precipitated and silver grows on the anode dendritically.
As silver migration evolves, silver grown dendritically short-circuits the anode and cathode and leak current increases. Inventions relating to suppressing silver migration are disclosed in Japanese Patent Laid-open Publications Nos. 2003-168823 and HEI-11-220171.
FIG. 15B is a cross sectional view of a semiconductor light emitting device disclosed in Japanese Patent Laid-open Publication No. 2003-168823. On a sapphire substrate 220, an AIN buffer layer 221, an n-type GaN layer 222, an InGaN luminous layer 223 and a p-type GaN layer 224 are stacked in this order. The p-type GaN layer 224 and luminous layer 223 are partially etched to expose a partial area of the n-type GaN layer 222. On the surface of the p-type GaN layer, an Ag layer 225 is formed being covered with a silicon oxide film 227.
A via hole is formed through the silicon oxide film 227 to expose a partial upper surface of the Ag layer 225. An Au layer 228 is formed on the silicon oxide film 227. The Au layer 228 is connected to the Ag layer 225 via the via hole formed through the silicon oxide film 227.
On the exposed surface of the n-type GaN layer 222, an n-side ohmic electrode 226 is formed which has a lamination structure of a V layer and an Al layer. Since the Ag layer 225 is covered with the silicon oxide film 227, Ag migration can be suppressed.
FIG. 15C is a cross sectional view of a semiconductor light emitting device disclosed in Japanese Patent Laid-open Publication No. HEI-11-220171. On a sapphire substrate 230, an AIN buffer layer 231, an n-type GaN layer 232, an n-type GaN clad layer 233, a luminous layer 234, a p-type GaN clad layer 235 and a p-type GaN contact layer 236 are stacked in this order. The lamination structure from the p-type GaN contact layer 236 to n-type GaN clad layer 233 is partially etched to expose a partial area of the n-type GaN layer 232.
An Ag layer 237 is formed on a partial surface of the p-type GaN contact layer 236. The Ag layer 237 is covered with a V layer 238 and an Al layer 239 so that Ag migration can be suppressed.
As disclosed in the above-described Japanese Patent Laid-open Publication No. 2003-168823 and Japanese Patent Laid-open Publication No. HEI-11-220171, Ag migration can be suppressed by covering the Ag layer used as the reflection film and electrode with an insulating film or another metal film. However, this migration suppression effect is not sufficient. It is desired to provide a semiconductor light emitting device capable of effectively suppress Ag migration.